In this paper Zanowski et al.
report the results of their study using the NOAA Geophysical Fluid
Dynamics Laboratory’s (GFDL) coupled climate model CM2G to explore the
role of the Weddell Sea polynyas in the establishment of deep-ocean
properties. Zanowski et al.
used statistical composite analysis of more than 30 polynya events that
occur in a preindustrial control run covering a period of 2,000 years to
quantify the temperature, salinity and water mass changes associated
with a composite event. Warming of between 0.002oC and 0.019oC
per decade occurs at a depth of below 4,200 m in the basins of the
Southern Ocean for
the time period following the cessation of the composite polynya, which
has been termed the “recovery”. Temperature and salinity changes were
found to be strongest near the region of polynya formation in the
Southern Ocean and the South
Atlantic Ocean. It was revealed by comparison of the model results
with observations of abyssal temperature that the Weddell Polynya
recovery in the 1970s could account for 10% ± 8% of the recent warming
in the abyssal Southern Ocean. This percentage is as little as 6% ± 11%
or as much as 34% ± 13% for individual Southern Ocean basins.

Conclusions

Changes in the abyssal ocean temperature and salinity that followed the
cessation of a composite Weddell Sea polynya in GFDL’s coupled climate
model CM2G were analysed. In
order to quantify warming of the deep ocean that occurred during the
recovery period 2 composites were constructed. Zanowski et
al. found patterns of warming
of deep basin below 4,200m of similar, but not smaller than, the
magnitude that has been reported previously (Purkey & Johnson, 2010). In
the South Atlantic and Southern Ocean basins the strongest warming
signals were found to occur which tended to decay with distance from the
Weddell Sea. Warming rates of between 0.002oC and 0.019oC
per decade occurred in the basins of the abyssal Southern Ocean during a
period of 19 years in the recovery. It is suggested by comparison with
previously published results (Purkey & Johnson, 2010) that 10% ± 8% of
the warming observed in the abyssal Southern Ocean could be explained by
the recovery of the Weddell Polynya. According to Zanowski et
al., however, in the
Australian-Antarctic basin warming does not appear to result from the
current polynya cycle. When the contribution of the Australian-Antarctic
basin is excluded, the Weddell Polynya could explain 6% ± 8% of the
warming that has been observed in the abyssal Southern Ocean. The
percentage contribution varies between basins and is as high as 34% ±
13% for the Australian-Antarctic basin and 33% ± 14% for the Argentine
basin. Percentages for the Weddell-Enderby basin is 6% ± 11% and for the
Amundsen-Bellingshausen basin7% ± 15%.

It is suggested by cooling in the Brazil and Angola basins that the
current polynya signal reached these basins, though the recovery had yet
to begin. Therefore Zanowski et
al. do not expect signals from the Weddell Polynya in these basins
to have contributed to warming between the mid-1990s and the mid-2,000s,
though could do so after this period. It is suggested it is possible
that some warming that has been observed in the Brazil basin from 2005
onward (Johnson et al., 2014)
results from recovery of the Weddell Polynya.

Analysis of basins in the Northern Hemisphere was not carried out
because of the lack of a discernible polynya signal from internal
variability. According to Zanowski et
al. it is possible for
polynya signals to reach these remote basins in less than 50 years by
planetary waves (Kawase, 1987; Nakano & Suginohara, 2002; Purkey &
Johnson, 2010; Hirabara et al.,
2012). It is indicated by recent studies that deep convection and
temperature changes of the deep ocean around Antarctica could impact the
Northern Hemisphere, the North
Atlantic in
particular, via changes to the Atlantic meridional overturning
circulation (AMOC) (Martin et
al., 2013, 2015; Patara &
Böning, 2014). An explanation of changes in the basins of the Northern
Hemisphere may be helped by further exploration of the propagation of
the polynya signal.

It is clear that the abyssal ocean was impacted by the Weddell Polynya
of the 1970s. Zanowski et al.
say the importance of understanding these transient features is
emphasised by their model Analysis, in particularly their spatial
structure and the time scales which they act on. It has been proven that
composite analysis is a useful tool in the study of the polynyas, though
even with more than 30 polynyas variability between individual events
complicated analysis of the signal. The robustness of the composite
signal was lowered by differences in the length of events, magnitude,
and the spatiotemporal influence of the polynyas which often combined.
As a result the analysis by Zanowski et
al. was limited to broad,
mean changes on a large scale. Zanowski et
al. plan to investigate the
effects of variability between polynyas by regularly forcing large
polynyas in the Weddell Sea in order to overcome these issues. The
differentiation between changes that are caused by the current polynya
cycle and those caused by previous polynya cycles could be helped by
control over the timing of polynyas.

Distinguishing between the polynya signal and that of anthropogenic
climate change requires the modelling of polynya dynamics be accurate,
as the recovery of the polynya is manifested as a warming of the deep
and bottom waters. The study by Zanowski et
al. employed a single model
to investigate open-ocean polynyas, so the results are tied to the
particular realisation of ocean dynamics within the model. The effects
of open-ocean (?ocean-open) polynyas across CMIP5 models by examination
of bottom property changes that result from reduced open-ocean
convection under anthropogenic climate change has been indirectly
investigated (Heuzé et al.,
2015). However, the study of Zanowski et
al. aims at understanding
impacts that result from models creating AABW through open-ocean
convection, rather than through shelf processes (Heuzé et
al., 2913) instead of a
comparison of open-ocean polynya dynamics and effects. The only
comprehensive studies in which impacts related to open-ocean convection
are compared across models that could be found by Zanowski et
al. was the study by Heuzé et
al. (2015). The understanding
of the sensitivity of polynya signals to model characteristics such as
resolution, background diffusivity, and overflow parameterisation, could
benefit from multimodel studies that investigate polynya effects.